Wind turbines may use a permanent magnet generator instead of an electrically excited synchronous or asynchronous generator to improve the system efficiency, in particular at partial load, and, in some embodiments, the annual energy production (AEP) of the wind turbine is increased. Generators have in some embodiments one or more stator windings, in which a rotating magnetic field of the magnets, in particular the permanent magnets, induce a voltage. The induced voltage is proportional to the rotational speed of a rotor of the generator and also proportional to the field of the permanent magnets.
In typical embodiments, the magnetization of the magnets used in a permanent magnet generator is temperature dependent. In particular, the magnetization of the permanent magnets decreases when the temperature increases. Due to the higher magnetization at lower temperatures, a higher voltage is induced in windings of a power generator at lower temperatures than at higher temperatures.
In typical usage, the generator of a wind turbine is electrically connected to a grid or to inverters, such that the output voltage of the generator is approximately constant. Typically, the inverter or other equipment, for example a circuit breaker or a transformer, is usually rated to a maximum current or voltage. Typically, wind turbines are started at low ambient temperatures with a low rotor speed, such that the induced voltage into the stator windings is not high enough to destroy the equipment of the wind turbine.